Issue 47

M. Peron et alii, Frattura ed Integrità Strutturale, 47 (2019) 425-436; DOI: 10.3221/IGF-ESIS.47.33

step of 0.01 mm, and the critical radius results to be equal to 0.13 and 0.12 mm for specimens tested at 0.1 and 0.5 s −1 , respectively. The small difference in radius is due to the material’s limited strain rates sensitivity below 10 s −1 [69]. As the strain energy density is proportional to the square of the applied stress, the predicted tensile strength of the notched specimens, σ UTS,predicted , has been estimated with

W

c

σ

=

(7) 

UTS, predicted

W

unit_load

in which W C

is the critical SED value (Eqn. (2)) and W unit_load

is the SED value determined by means of FE analyses applying a unit

load. The SED prediction of the tensile failure for both the strain rates are reported in Tab. 3. Specimen Geometry Strain Rate (s -1 ) Experimental Data (MPa) SED Prediction (MPa)

Deviation (%)

Deep

0.1 0.5 0.1 0.5 0.1 0.5

127 129 132 135 119 123

118 124 132 139 118 124

-6.8

-4

Moderate

-0.1

+3.3

Razor

-0.8

+0.8

Table 3 : Prediction of tensile failure of moderate, deep and razor specimens using the SED approach.

For the geometries and test conditions reported in [60], the presented approach provides suitable prediction of tensile failure, where deviations (computed as the relative difference between the experimental and predicted values, and expressed in percentage) are generally lower than 4%, for predictions of moderate and razor grooved merely even below 1%. Impacts of the corrosive test environment were successfully accounted for. a) b)

Figure 6 : Determination of the SED critical parameters: (a) The value of the critical radius R C

has been varied with a step of 0.01 mm

and (b) with a finer step of 0.001 mm. Fatigue loading

As mentioned earlier, neither the NSIF fatigue threshold nor the fatigue limit of the plain material were available, and thus Eqns. (5) and (6) could not be applied to determine the critical SED parameters. An alternative approach has thus been used using Ansys ® : the radius of the control volume has been varied until the SED values for two different specimen geometries, at the same number of cycles, were equal. In particular, circumferentially razor-grooved dog-bone and U notched specimen, with a notch root radius of 0.45 mm, have been modelled, and the critical radius has been varied from 0.01 to 0.1 mm, with a step of 0.01 mm (Fig. 6a). Then, in the range between 0.06 and 0.07 mm, where the difference

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